This invention relates to the field of photoluminescence (PL) analysis, in which a light source is used to excite a sample, and the photons emitted by the sample are passed through a spectrometer which provides desired spectral information.
Although its potential uses ae much broader, the primary concern of the present invention relates to the determination of impurity concentrations in single crystal silicon (Si), whether such impurities are intentionally or unintentionally present in the silicon. These impurity determinations are an important means of evaluating the characteristics of electronic devices in integrated circuit chips. The use of PL analysis is essentially directed at determining surface characteristics, whereas other types of analysis ae used in determining bulk silicon characteristics.
In U.S. patent application Ser. No. 411,603, also filed by the present applicant (on Aug. 26, 1982), abandoned, the use of the PL technique for surface analysis of silicon crystal impurities is discussed at length. Both the value of this technique, and the problem of getting sufficient radiation throughput to the detector, are pointed out in that application.
Even with the improvements disclosed in Application Ser. No. 411,603, the PL analysis of silicon chip impurities is subject to significant deficiencies. The present application is intended to deal with those deficiencies by providing a fundamentally different approach to the problem of PL analysis of silicon (and potentially other materials).
The wavelength range of the radiation which needs to be measured is in a very "awkward" part of the spectrum. It covers approximately the range of 1.07 through 1.127 microns, which makes the system subject to all the measurement problems of both the visible and infrared portions of the spectrum.
A major problem is the wavelength limitation of the available photomultiplier (PM) detector tubes. These tubes, such as the S1 photo-cathode tube, have a very desirable signal-to-noise ratio because of their large, essentially noise-free, amplification. But there sensitivity begins to fall off rapidly in the portion of the spectrum which is particularly relevant for PL analysis of silicon surface impurities. FIG. 1 of the drawings is a graph which shows the response characteristics of the available PM tubes as a dashed line curve A. The wavelengths are plotted on the X-axis and the responsiveness of the tubes is plotted on the Y-axis. As is apparent from FIG. 1, the signal from the S1 tube, which is the best available PM tube, begins to drop off rapidly at a wavelength of about 0.9 microns.
In order to provide a detector having a wavelength range which comfortably includes the significant portion of the spectrum in silicon PL analysis, the PM tube and its amplification (or gain) advantage apparently need to be eliminated. Substituting a photo-voltaic detector having the desired wavelength characteristics, such as germanium, eliminates the signal-to-noise benefits derived from the PM tube's high noise-free internal amplification.
Reduction of the usable signal in a system of the type disclosed in Aplication Ser. No. 411,603 would seriously aggravate the radiation throughput deficiency inherent in PL silicon analysis systems having grating monochromators. So a means of substantially increasing the radiation throughput between the sample and the detector is necessary for advancement of the art of PL analysis. This application, and its parent application, Ser. No. 555,607, provide solutions for the problems indentified above.
Furthermore, the continuing use of the system disclosed in Application Ser. No. 555,607 has resulted in such a large and unexpected improvement in PL analysis that heretofore unsuspected limitations, or deficiencies, in monochromator systems have become apparent. Three major deficiencies, previously unsuspected, have now become apparent.
One such deficiency in monochromator PL systems is due to the extremely rapid diffusion of the excitons in the semiconductor crystal. The excitons are transient entities in the crystal, which are caused by the laser excitation photons, and which, in turn, cause emission of photons by the crystal. Because of the exciton diffusion, and consequent wider spatial distribution diffusion of the crystal-emitted photons, a monochromator used for PL analysis is, in fact, doomed to inadequacy because it cannot capture enough of the crystal-emitted light. The characteristics of excitons in semiconductor crystals are discussed in an article titled "Excitonic Matter" by Wolfe and Mysyrowicz in the March, 1984 issue of Scientific American (pages 98-107). This article states that "each laser pulse creates a new cloud of excitons that diffuses and decays before the next pulse is generated". The article further explains that "the excitons diffuse like a dilute gas of atoms. The diffusion constant for an exciton gas, however, is much larger than it is for an ordinary atomic gas. (text omitted) The extremely fast diffusion of the exciton is a result of its small mass and its relatively infrequent scattering by other particles at low temperatures."
A second deficiency in monochromator PL systems, which was not perceived until the present invention was developed, is the absence of acceptable calibration, i.e., comparability of results from sample to sample. As a result of a spectacular improvement in sensitivity, i.e., signal-to-noise ratio, attained by using the apparatus of the present invention, as well as the extended spectral coverage, it is now apparent that the "electron hole droplet" (EHD) phenomenon has heretofore effectively destroyed sample-to-sample calibration. The EHD phenomenon is discussed in the previously cited article and also in an article titled "Onsets of the EHD Luminescence in Si" by Hammond and Silver in Physical Review Letters (American Physical Society), Volume 42, Number 8 (Feb. 19, 1979, pages 523-526). The latter article predicts, by extrapolation, the point of onset of EHD luminescence as laser intensity on the semiconductor surface is increased. The significance of the EHD effect relates to the excitonic gas density induced in the sample. This gas density has been seen to vary dramatically from sample to sample for a given laser excitation level, affecting the relative intensities of the free exciton to the bound exciton lines. Since the quantitative measurement of impurity concentration is calculated from these relative intensities, the changing excitonic gas density effectively destroys the calibration of the measurement.
A third deficiency in monochromator PL systems, which the present invention spotlights, is the limitation of results forced by the need to largely sacrifice certain parameters as a trade-off for those considered vital. Specifically, in spectrometry, there are four parameters which represent desirable goals--(1) sensitivity, (2) speed of data acquisition, (3) resolution, and (4) spectral coverage. In monochromator PL systems, it is necessary to maximize sensitivity and speed of data acquisition. It has, therefore, been necessary for such systems to accept whatever is available in the other two parameters--resolution and spectral coverage. This has severely limited the information produced by monochromator PL systems, to an extent not even suspected until the advent of the present invention.